1. Introduction to OSPF
• Open Shortest Path First (OSPF) is a link-state
routing protocol based on open standards
– The Open in OSPF means that it is open to the
public and is non-proprietary

2. More on OSPF…
• OSPF can be used and configured as a single area for
small networks
– can also be used for large networks if hierarchical network
design principles are used (multi-area OSPF)
• What does a hierarchical design mean?
– Multiple areas connect to a distribution area, area 0, also
called the backbone
– Allows for extensive control of routing updates
– Defining areas…
• reduces routing overhead, speeds up convergence, confines
network instability to an area and improves performance.

3. Multi-Area OSPF

4. OSPF Terms - Link

5. OSPF Terms – Link-State

6. OSPF Terms –
Link-State Database
Every router in the same
OSPF area will have the
same link-state database

7. OSPF Terms - Area
Internal
Routers
Border
Router

8. OSPF Terms – Cost

9. OSPF Terms – Forwarding Database
(Routing Table)
The lowest cost path is
added to the routing table

10. OSPF Terms –
Adjacencies Database

11. OSPF Terms – Designated Router (DR) and
Backup Designated Router (BDR)
The DR and BDR serve
as focal points for routing
information exchange
DROTHERs

12. Establishing a Neighbor Relationship with
Other Routers
• A neighbor relationship is required for OSPF routers to share
routing information
• A router will try to become adjacent, or neighbor, to at least one
other router on each IP network to which it is connected (Some
routers may try to become adjacent to all their neighbor
routers)
• Other routers may try to become adjacent to only one or two
neighbor routers
• OSPF routers determine which routers to become adjacent to
based on the type of network they are connected to
• Once an adjacency is formed between neighbors, link-state
information is exchanged

13. Types of OSPF Networks

14. Election of DR and BDR
•The DR acts as the spokesperson for the segment
•All other routers on the segment send their link-state
information to the DR

15. DR and BDR Election as it relates to
network types
• The DR sends link-state information to all other routers on the segment using
the multicast address of 224.0.0.5
• Disadvantage of DR election…the DR represents a single point of failure…
therefore:
– A second router is elected as a backup (BDR) in case of DR failure
– To ensure that both the DR and the BDR see the link states all routers send
on the segment, the multicast address for all designated routers, 224.0.0.6, is
used.
• On point-to-point networks only two nodes exist and no DR or BDR is elected
(routers become fully adjacent with each other)

16. The Hello Protocol
• When a router starts an OSPF routing process on an
interface, it sends a hello packet and continues to
send hellos at regular intervals
– The rules that govern the exchange of OSPF hello packets
are called the Hello protocol
– Type field is set to 1 to indicate packet contains hello
information

17. More on Hello Packets…
• Hello packet timing…
– sent every 10 seconds by default on broadcast multi-
access and point-to-point networks
– 30 seconds by default on interfaces that connect to
NBMA networks, such as Frame Relay

18. Single Area OSPF Operations – Step 1:
Neighbor Discovery

19. Single Area OSPF Operations – Step 2: Elect DR &
BDR on Multi-Access Networks
We’ll get to what determines which router becomes the DR
and which becomes the BDR later…

20. Single Area OSPF Operations – Step 3:
Selecting the Best Route

21. Single Area OSPF Operations – Step 4:
Maintaining Routing Information

22. Basic OSPF Configuration
• To enable OSPF routing, use the global configuration
command syntax:
– Router(config)#router ospf process-id
• The process ID is a number that is used to identify an
OSPF routing process on the router
– Multiple OSPF processes can be started on the same router
– Process ID can be any value between 1 and 65,535
– Most network administrators keep the same process ID
throughout an autonomous system (not a requirement)
– It is rarely necessary to run more than one OSPF process on a
router

23. Basic OSPF Configuration
• IP networks are advertised as follows in OSPF:
– Router(config-router)#network address
wildcard-mask area area-id
• Each network must be identified with the area to which
it belongs
– The network address can be…
• a whole network
• a subnet
• or the address of the interface
• The wildcard mask represents the set of host addresses
that the segment supports

24. OSPF Priority & DR and BDR Election
• A router with the highest OSPF priority will be selected as
the DR
– If the network type of an interface is broadcast, the default
OSPF priority is 1
– The priorities can be set to any value from 0 to 255
– Router with the second highest priority will be the BDR
– When OSPF priorities are the same, the OSPF election for DR is
decided on the router ID (highest ID wins)
• To modify the OSPF priority on an interface…
– Router(config-if)#ip ospf priority number
• To verify OSPF operation…
– Router#show ip ospf interface type number

25. OSPF Router ID
• When the OSPF process starts, the Cisco IOS uses
the highest local active IP address as its OSPF
router ID
– If there is no active interface, OSPF process will not
start
– If the active interface goes down, the OSPF process
has no router ID and therefore ceases to function until
the interface comes up again

26. Loopback Interface
• To ensure OSPF stability there should be an active interface for the
OSPF process at all times.
– A loopback interface, which is a logical rather than a physical interface,
can be configured for this purpose
• When a loopback interface is configured, OSPF uses this address as the
router ID, regardless of the value
• On a router that has more than one loopback interface, OSPF takes the
highest loopback IP address as its router ID.

27. Modifying the Cost Metric
• OSPF uses cost as the metric for determining the best
route
• Cost is calculated using the formula:
– 108
/bandwidth (bandwidth is expressed in bps)
• The Cisco IOS automatically determines cost based on
the bandwidth of the interface
– You must set the correct interface bandwidth by:
• Router(config)#interface serial 0/0
Router(config-if)#bandwidth 64
• The default bandwidth for Cisco serial interfaces is 1.544
Mbps, or 1544 kbps.

28. More on OSPF Cost
• Cost can be changed to influence the outcome of the OSPF cost
calculation
• Use the following interface configuration command to set the
link cost:
– Router(config-if)#ip ospf cost number
• The cost number can be between 1 and 65,535

29. Configuring OSPF Authentication
• Each OSPF interface can present an authentication
key for use by routers sending OSPF information to
other routers on the segment
• The authentication key, known as a password, is a
shared secret between the routers
• This key is used to generate the authentication data
in the OSPF packet header
• Passwords can be sent as plain text or encryted

30. For Simple Authentication – Plain Text
• Use the following command syntax to configure OSPF
authentication:
– Router(config-if)#ip ospf authentication-
key password
• After the password is configured, authentication must
be enabled:
– Router(config-router)#area area-number
authentication
• Simple Authentication can be easily decoded if a
packet sniffer captures an OSPF packet

31. Encrypted Authentication
• Use the interface configuration command mode
syntax:
– Router(config-if)#ip ospf message-
digest-key key-id md5 encryption-
type key
• The following is configured in router
configuration mode:
– Router(config-router)#area area-id
authentication message-digest

32. OSPF Timers (Hello Interval and Dead
Interval)
• OSPF routers must have the same hello intervals and same dead
intervals to exchange information (very important!)
• By default, the dead interval is four times the value of the hello
interval
– Means a router has four chances to send a hello packet before
being declared dead
• On broadcast OSPF networks…
– the default hello interval is 10 seconds and the default dead
interval is 40 seconds
• On nonbroadcast networks…
– the default hello interval is 30 seconds and the default dead
interval is 120 seconds
• These default values result in efficient OSPF operation and
seldom need to be modified

33. Configuring OSPF Timers
• To configure the hello and dead intervals on an
interface, use the following commands:
– Router(config-if)#ip ospf hello-
interval seconds
Router(config-if)#ip ospf dead-
interval seconds

34. Configuring a Default Route
• A configured default route is used by a router
to generate a gateway of last resort:
– Router(config)#ip route 0.0.0.0
0.0.0.0 [interface | next-hop
address]
• The following configuration statement will
propagate this route to all the routers in a
normal OSPF area:
– Router(config-router)#
default-information originate

35. Verifying OSPF Configuration

36. Debug & Clear Commands